1. Field of the Invention
The present invention relates to a device for estimating an amount of intake air of an internal combustion engine.
2. Description of the Related Art
In order to control the air-fuel ratio, it is necessary to know the amount of intake air supplied into the cylinder. The amount of the intake air has been detected by an air flow meter arranged upstream of the throttle valve or has been calculated based on the intake pipe pressure detected by a pressure sensor arranged downstream of the throttle valve. However, the air flow meter and the pressure sensor have a delay in the response and thus are not capable of correctly detecting or calculating the amount of the intake air during transient conditions of the engine.
To correctly detect the amount of the intake air even during transient conditions of the engine, it has been proposed to calculate the intake pipe pressure (Pm) and to estimate the intake air amount (mc) based on the calculated intake pipe pressure (Pm), as disclosed in Japanese Unexamined Patent Publications No. 2002-201998 or No. 2001-41095.
To calculate the intake pipe pressure (Pm), in general, a relation formula between the intake pipe pressure (Pm) and the amount (mt) of the air passing through the throttle valve is determined by modeling the intake pipe. This relation formula is transferred to a discrete formula, and the present intake pipe pressure (Pm(i)) is calculated based on the intake pipe pressure (Pm(i−1)) at the last time and the amount (mt(i−1)) of the air passing through the throttle valve at the last time. Upon thus calculating the present intake pipe pressure (Pm(i)), the present intake air amount (mc(i)) can, then, be estimated based thereupon.
In such an estimation of the amount of intake air (mc), a nonrealistic intake pipe pressure (Pm) greater than the atmospheric pressure (Pa) can be calculated. In such a case, the present intake pipe pressure (Pm(i)) is replaced by the atmospheric pressure (Pa), and the amount of intake air (mc(i)) is estimated.
In the above-mentioned related art, the intake pipe pressure (Pm) replaced by the atmospheric pressure (Pa) is used in the next time as the intake pipe pressure (Pm(i−1)) at the last time to calculate the present intake pipe pressure (Pm(i)). Even if the intake pipe pressure (Pm) is simply limited by the atmospheric pressure, however, the factors making the intake pipe pressure (Pm) be calculated higher than the atmospheric pressure have not been excluded. Unless these factors are excluded, it is probable that the amount of intake air (mc) may be incorrectly estimated after the intake pipe pressure (Pm) is limited.